Server: SuperMicro AMD Opteron 1210 Dual-Core
OS: Solaris 10u2 x86
RAM: 4 Gb
Mail Spool: Maildir on compressed ZFS
Users Maildir Size: 1-500MB per user
Dovecot version: 1.0.3
MTA: Postfix 2.4 with Dovecot Local Delivery Agent
Client Software: Thunderbird, Evolution, RoundCube Webmail, Nokia Smartphones
Dovecot and Postfix runs in their own Solaris Zone with access to users home directories which are compressed ZFS on top of hardware RAID-5. Dovecot Indexes are placed outside home directories on RAID-10 spool, file system is again compressed ZFS. Compress ratio for indexes is 1.86x, for mail it is anything between 1.0 - 2.0x. Accounts are stored in MySQL database. Dovecot is compiled with Sun Studio 12.
Yes, system is a bit over engineered.